Reduction of corrosion in the electrolyte surrounding a multipolar cell

ABSTRACT

IN THE PRODUCTION OF SODIUM CHLORATE OR PERCHLORATE A MEHOD OF REDUCING CORROSION OF ELECTRICALLY CONDUCTIVE PARTS, PARTICULARLY COOLING COILS, IN A TANK SURROUNDING A MULIPOLAR ELECTROLYTIC CELL, SAID TANK CONTAINING AN AQUEOUS SOLUTION OF SODIUM CHLORIDE AS ELECTROLYE FROM WHICH THE ELECTROLYTE IS CONTINUOUSLY RECIRCULATED THROUGH THE CELL WHICH METHOD COMPRISES INTIMATELY ADMIXING THE EFFLUENT ELECTROLYTE FROM EACH CELL UNIT FROM THE CELL PRIOR TO ITS PASSAGE TO THE TANK WHEREBY TO OBTAIN EQUALIZATION OF THE ELECTRIC POTENTIAL IN THE EFFLUENT.

March 2, 1971 CRANE 3,561,503

REDUCTION OF CORROSION IN THE ELECTROLYTE SURROUNDING A MULTIPOLAR CELL Filed March 5, 1968 Hi \v\ w v I I I I r l m I l m l I I U m 1 a Hlfi L [.l I I N at I i g I flrrae IVE x s United States Patent 888,37 Int. Cl. C01b 11/26; C2211 ]/02 US. Cl. 204-95 3 Claims ABSTRACT OF THE DISCLOSURE In the production of sodium chlorate or perchlorate a method of reducing corrosion of electrically conductive parts, particularly cooling coils, in a tank surrounding a multipolar electrolytic cell, said tank containing an aqueous solution of sodium chloride as electrolyte from which the electrolyte is continuously recirculated through the cell which method comprises intimately admixing the efiiuent electrolyte from each cell unit from the cell prior to its passage to the tank where-by to obtain equalization of the electric potential in the efliuent.

The present invention relates to the operation of a multipolar electrolytic cell particularly in the production of sodium chlorate or perchlorate by the electrolysis of an aqueous solution containing sodium chloride and in particular relates to an improvement in the operation of such a multipolar cell which results in the reduction of the corrosion of the electrically conductive parts and in particular those parts fabricated from metal such as electrically conductive cooling coils into the tank into which the electrolyte is passed for the subsequent chlorate producing reaction to occur.

This application is a continuation-in-part application of US. application No. 362,718 filed Apr. 27, 1964 now abandoned.

As is well known in the art a multipolar electrolytic cell is an electrolytic cell which consists of a number of cell units which are electrically connected in series but through which the electrolyte flows in parallel. It is formed from a pair of monopolar electrodes which have the same polarity on all surfaces and through which surfaces current enters or leaves at the same polarity relative to the surrounding electrolyte. Dividing the space between the monopolar electrodes are located a plurality of spaced intermediate bi-polar electrodes which are at a positive polarity on one side and a negative polarity on the other side with respect to the electrolyte. In such a cell the total voltage impressed between the two monopolar electrodes, which is typically of the order of 100 volts is taken up by the bi-polar electrodes such that the sum of the difference between the voltage on adjacent bi-polar electrodes is equal to the aforesaid total voltage. Thus, for instance when there is a difference of four volts across each pair of adjacent electrodes (which form a cell unit) there would be 24 bi-polar electrodes which would give 25 individually separate electrolytic cell units between each pair of monopolar electrodes, each of the cell units operating at a different voltage. Thus, in a multipolar electrolytic cell there is a voltage across the electrolyte as it enters and leaves each individual cell unit and a current flow or loss through each inlet or outlet.

In the electrolysis of an aqueous solution of sodium chloride (brine) in such a multipolar cell to produce sodium chlorate and sodium perchlorate it is conventional to pass the electrolyte effluent for each cell unit to a common tank where the relatively slow chlorate production reaction involving the combination of hypochlorous ac1d and hypochlorite ions according to the equation takes place. The hypochlorous acid and hypochlorite ions are generated by relatively fast reactions occurring in the electrolytic cell. From the tank the electrolyte is recirculated through the cell for further electrolysis until the desired concentration of chlorate or perchlorate is achieved in the tank liquor when the liquor is withdrawn from the tank and sent for further processing for the recovery of sodium chlorate or perchlorate.

As aforesaid the outflow of the electrolyte from the cell units forming the multipolar cell which are at different voltages is discharged into a common tank which acts as a reaction vessel and in which tank the multipolar cell is usually located. In such a tank any electrically conducting part inserted in the electrolyte before it is well mixed and separated from the main body of the electrolyte in the cell will conduct electricity alternatively to the electrolyte because the conducting part is usually a better conductor than the electrolyte itself and if not a better conductor will at least conduct a portion of the current inversely proportional to its resistance and as such corrosion takes place on the electrically conductive parts due to accelerated galvanic action.

It is common practice in the operation of multipolar cells to have present in the tank heat exchangers to remove the heat from the electrolyte which has been generated by the inefficiencies of the electrolytic process and the reaction of the chemical intermediates. The most economic heat exchangers use cooling water and therefore require a relatively high thermal conductivity and such heat exchangers are therefore usually made of copper, aluminum, iron, graphite, silver or other metals all of which it will be seen have relatively good electrical conductivity. When such cooling coils formed of these materials are inserted in the tanks into which the eflluent electrolyzed liquor is passed from the multipolar cell there is substantial corrosion due to accelerated galvanic action. Thus, the normal rate of corrosion of these parts is increased by the many stray electric currents flowing through the electrolyte which short circuit through these conducting parts. Thus, measurements taken with probes, insulated except for a small section at the point of measurement, show a difference of electric potential between electrolyte sections or domains in the tank containing the electrolyte. In fact it has been found that by moving a test probe a few inches, a difference of as much as 2 millivolts will occur between varying sections of electrolyte in the tank and this is suflicient to substantially increase the corrosion of electrically conductive parts in the tank. In practice it is found that the coils and other conduction points will corrode particularly quickly at corners or exposed parts, at supports, metal junctions or welds. This enhanced corrosion effect on such electrically conductive parts has been reduced heretofore by hanging sacrificial conduction bars or screens around or adjacent to the electrically conductive parts e.g. the cooling coils or by erecting an oversized tank so that the various stray liquid potentials have time to be equalized before reaching the coils. However, such attempts have not been entirely successful and have many disadvantages.

The present invention provides an improvement in the operation of multipolar electrolytic cells particularly in the production of sodium chlorate and perchlorate including the electrolysis of an aqueous brine solution in which the corrosion of electrically conductive parts in the tank from which the electrolyte is withdrawn or returned to after electrolysis in the multipolar electrolytic cell is substantially reduced and in a preferred embodiment eliminated.

In its broadest aspect the present invention provides in the operation of a multipolar electrolytic cell in which the effluent electrolyte from a plurality of cell units formed by the electrodes in said cell is continuously passed to a tank for reaction of the chemical intermediates formed during said electrolysis and electrolyte from said tank continuously recirculated upwardly through said cell units during which passage the electrolyte in each cell unit is subjected to electrolysis at a different electrical potential; a method of equalizing the electric potential in said effluent electrolyte which comprises collecting the effluent immediately on emergence from said cell units and intimately mixing said effluent before passage into said tank whereby to substantially reduce corrosion of electrically conductive parts in said tank.

In a particular embodiment thereof the present invention provides in the operation of a multipolar electrolytic cell for the production of sodium chlorate or perchlorate by the electrolysis of an aqueous solution containing sodium chloride in which the effluent electrolyte from a plurality of cell units formed by the monopolar and bipolar electrodes in said cell is continuously passed to a tank for reaction of the chemical intermediates formed during the electrolysis to form such chlorate or perchlorate and electrolyte from said tank continuously recirculated upwardly through said units during which passage the electrolyte in each cell unit is subjected to electrolysis at a different electric potential, a method of equalizing the electric potential in said effluent electrolyte which comprises collecting the effluent electrolyte immediately on emergence from said cell units and intimately mixing said effiuent electrolyte before passage into said tank whereby to substantially reduce corrosion of electrically conductive parts in said tank.

As is conventional the tank suitably surrounds the multipolar electrolytic cell, in a particularly preferred embodiment of the present invention, therefore, there is provided in the operation of a multipolar electrolytic cell for the production of sodium chlorate or perchlorate by the electrolysis of an aqueous solution containing sodium chloride in which electrolyte is continually passed from a tank surrounding said cell upwardly through the cell units formed by the monoand bi-polar electrodes in said cell, during which passage the electrolyte in each cell unit is subjected to electrolysis at different electric potentials and the electrolyzed electrolyte is returned to said tank, for reaction of the chemical intermediates to form said chlorates or perchlorate, a method of equalizing the electric potential in the effluent electrolyte from said cell which comprises collecting the effluent on emergence from said cell in a container intimately mixing the effiuent in said container and subsequently passing the mixed effiuent to said tank whereby to substantially reduce corrosion of parts which are electrically conductive, in particular the cooling coils in said tank.

The present invention also provides in a system comprising a multipolar electrolytic cell, a tank for containing an electrolyte surrounding said cell, inlets adjacent the base of said cell and outlets adjacent the top of said cell each cell unit of said cell being provided with an inlet and outlet whereby electrolyte may be continuously passed from said tank upwardly through each cell unit and the electrolysed electrolyte continuously returned to said tank for reaction of the chemical intermediates formed during said electrolysis, the improvement which comprises a container arranged to collect the electrolyte effluent from said outlets for intimate mixing thereof and means for passing the electrolyte effluent from said container to said tank.

The present invention also provides in a particular embodiment thereof in a system for the production of sodium chlorate or perchlorate by the electrolysis of an aqueous solution containing sodium chloride comprising a multipolar electrolytic cell, a tank for containing electrolyte surrounding said cell, inlets adjacent the base of said cell and outlets adjacent the top of said cell, each cell unit of said cell being provided with an inlet and outlet whereby electrolyte may be continuously passed from said tank upwardly through each cell unit and the electrolysed electrolyte continuously returned to said tank, the improvement which comprises a container arranged to collect the electrolyte effluent from said outlets for intimate mixing thereof, and means for passing the electrolyte effluent from said container to said tank.

The mixing of the electrolyte effluent from the cell units is suitably effected in a container and this in a preferred embodiment of the present invention comprises a trough which is attached to one or both sides of the cell above the level of the electrolyte in the tank and beneath the outlets from the cell. In order to return the mixed effluent from the trough to the tank a downpipe or channel is suitably provided from the trough which besides returning the effluent to the tank effects further mixing of the effiuent and thus, enhances the equalization of the electric potential in the efiluent.

In contrast to the multipolar electrolytic cell with which the present invention is concerned in a monopolar cell each cell is an intimate unit in its own tank and each cell unit has a separate flow in and out of electrolyte and a separate flow of electricity from and to each cell unit and, thus a monopolar cell is operated at low voltage and high current while a multipolar cell unit operates at relatively high voltage and relatively low current. Further, in a monopolar cell the tank and cooling coils or surfaces are at cathode potential, and hence electrically protected from accelerated galvanic or electrolytic corrosion. This is not possible in a multipolar cell because there are many separate cathodes, each at different voltages to the container and cooling surfaces. In a monopolar cell, the maximum overall voltage difference is that of one cell (approximately 3.5 volts) whereas in a multipolar cell the difference is as much as 60 volts plus or minus from the average. Thus, the aforesaid corrosion problems which arise with respect to a multipolar cell do not occur in a monopolar cell operation.

The present invention will be further illustrated by way of the accompanying drawing in which the single figure is a perspective view partially broken away of a multipolar electrolytic cell modified according to a preferred embodiment of the present invention located in a tank.

Referring to the drawing a multipolar electrolytic cell 1 containing an electrolyte 2 comprising an aqueous solution containing sodium chloride has disposed therein monopolar electrodes 3 and intermediate bipolar elecrodes 4. As is conventional with multipolar cells the total potential is impressed between the monopolar electrodes 3 by means of copper leads 3a and the bipolar electrodes 4- assume a voltage such that the sum of the differences between each adjacent electrode, which forms a cell unit, totals that between the monopolar electrodes 3.

The cell 1 is located on a block 5 in a tank 6 containing electrolyte 2 which is circulated through the cell 1 via inlets 7 adjacent the bottom of the cell 2 and an outlet 8 adjacent the top of the cell 2 above the level of the electrolyte 2 in the tank 6, each inlet 7 and outlet 8 communicating directly with a cell unit of the cell 1. The effluent electrolyte 2 in such a system which is warm and contains gas bubbles and stray charges spreads out over the surface of the electrolyte 2 in the tank 6 causing corrosion in the electrically conductive parts in the tank 6 particularly the metals parts due to the differences in potential in the different portions or domains of the effluent electrolyte. In particular in order to remove heat from the effluent electrolyte in the tank 6 due to the heat generated in the electrolysis process, cooling coils 9 are provided through which a coolant, such as water, passes which are located in the tank 6 and are usually formed from an electrically and thermally conductive material, such as a metal, and thus, tend to corrode rapidly in the operation of the cell 1. Alternatively, to the cooling coils 9, to effect cooling of the electrolyte the efiluent may pass through cooling coils immersed in a coolant such as water, on passage to the tank 6, where similar corrosion problems will arise in the cooling coils.

As shown in the figure according to a preferred embodiment of the present invention the outlet pipes 8 discharge into troughs 10 which ensures intimate mixing of the effluent electrolyte from each of the cell units of the cell 1 and as aforesaid the effluent from each cell unit which is at a different electrical potential is intimately admixed and equalization of the electric potential in the efiiuent is achieved. From the troughs 10 which are suitably made from an electrically non-conductive, i.e. corrosion resistant material the effluent passes through a downpipe 11 to the tank 6 which pipe 11 further enhances mixing of the efiiuent electrolyte.

The present invention will be further illustrated by way of the following examples.

EXAMPLE I Using the cell shown in the accompanying drawings three aluminum pipes were taken each having a length of four feet and a diameter of 2 inches. A first of said pipes was disposed along the trough 10 on one side of the cell 1 twelve inches from the outlets 8, a second pipe was disposed along the trough 8 on the other side of the cell 1 six inches from the outlets 8 and the third of said pipes was disposed parallel to and at the same height as the other two pipes one-half inch outside the trough 8. Thus, these pipes simulate cooling coils which are normally present in the operation of the cell 1. After 28 days operation of the cell the first pipe inside the trough 8 was reduced in weight by 24% and was perforated over about of its surface and was useless as a conductor of water. The third pipe outside the trough 8 lost one percent of its weight in the same period and showed minimal corrosion. The second pipe inside the trough 8 lost 28% of its weight in 28 days and was useless as a water conductor. It is easily seen that the large difference in corrosion of the pipes inside and outside the trough 8 showed the effect of corrosion reduction due to the presence of the trough 8.

EXAMPLE II Using the cells shown in the accompanying drawing a similar aluminum pipe as in Example I twelve inches long was laid in the bottom of the trough 8 of the cell 1 and another piece twelve inches long put in the liquor emerging from the downpipe 11 after the electrolyte passes from the individual cell units has fully mixed. After 28 days the aluminum pipe in the trough 10 had lost 22% of its weight and the aluminum pipe in the electrolyte beneath the downpipe 11 had lost 2% of its weight again showing the effect of the corrosion reduction due to the trough 11.

I claim:

1. In the method of operation of a multipolar electrolytic cell having a plurality of spaced, bipolar electrode plates for the production of sodium chlorate or perchlorate by the electrolysis of an aqueous solution containing sodium chloride said cell provided with inlet means and outlet means and a tank surrounding said cell in fluid communication therewith in which electrolyte is continuously passed from said tank surrounding said cell upwardly through the cell units formed by the electrodes in said cell during which passage the electrolyte in each cell unit is subjected to electrolysis at a different potential and the electrolyzed electrolyte is continuously returned to said tank for reaction of the chemical intermediates formed during said electrolysis to form such chlorate and perchlorate; the improvement consisting of a method to substantially reduce corrosion of electrically conductive parts in said tank by equalizing the potential in the efiluent electrolyte from the cell which comprises collecting the effluent electrolyte on emergence from said cell in a container set within said tank in fluid communication with said cell contained within said tank, intimately mix ing the effluent in said container and subsequently passing the mixed eflluent to said tank surrounding said cell.

2. A method as claimed in claim 1 in which the efiluent is collected and admixed in a corrosion resistant trough located above the level of the electrolyte in the tank and the electrolyte is passed from the trough to the tank through a downpipe.

3. A method as claimed in claim 1 in which the effluent is collected and admixed in a corrosion resistant container located above the level of the electrolyte in the tank.

References Cited UNITED STATES PATENTS 2,731,325 l/l956 Kesting 23-152 2,350,669 6/1944 Boller 204-237 718,249 l/1903 Haas 204268 3,219,563 11/1965 Collins et al.

FOREIGN PATENTS 7,353 1/1896 Sweden 204 OTHER REFERENCES General Chemistry, Sisler, Vanderwerf & Davidson, 1949, p. 426.

JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner U.S. Cl. X.R. 204-269 Disclaimer 3 567,603.-Geo'r e J. 0mm, Islington, Ontario Canada. REDUCTION OF SION IN THE ELEGTROLYTE SURROUNDING A MULTIPOLAR CELL. Patent dated Mar. 2, 1971. Disclaimer filed Oct. 29, 1970, by the assignee, Huron-Nassau Ltd. Hereby disclaims the terminal portion of the term of said patent subsequent to Mar. 31, 1987.

[Ojficz'al Gazette J1me 1, 1.971.] 

